1. Field of the Invention
The present invention relates to a toner replenishing device and an image forming apparatus including the toner replenishing device.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2004-139031 discloses a conventional toner replenishing device, in which toner supplied from a toner bottle is temporarily housed in a toner container. Then, the toner in the toner container is supplied to a developer container of a developing unit that performs development by a toner replenishing conveying member. The toner replenishing conveying member rotates to convey the toner in the toner container.
A main object of the replenishment of toner in the above toner replenishing device is to replenish an amount of toner that has been consumed by image outputting to keep toner density in the developer container of the developing unit. However, if an amount of toner in the toner container of the toner replenishing device becomes smaller and a stable amount of toner cannot be supplied, toner density in the developer container of the developing unit is lowered. This causes formation of an image of which image density is lowered.
The conventional toner replenishing device includes a toner detection sensor that detects whether toner remains at a certain height in the toner container. The toner replenishing device can detect reduction of toner in the toner container based on a detection result of the toner detection sensor. Therefore, the toner replenishing device can detect a state where a toner bottle is empty but toner in the toner container remains (hereinafter, referred to as “near empty”). The toner bottle can be replaced before toner in the toner container is completely consumed by replacing the toner bottle in the near empty state. Therefore, toner can be stably replenished. This can prevent image density from being lowered because of the lowered toner density in the developer container.
Japanese Patent Application Laid-open No. 2004-220012 discloses an image forming apparatus including another conventional toner replenishing device. The conventional toner replenishing device includes a toner agitation member that rotates to agitate toner in a toner container that temporarily contains the toner so that the toner does not aggregate in the toner container. In the toner replenishing device, driving force is supplied to a toner replenishing conveying member and the toner agitation member from a common toner replenishing operation driving source through gears.
The recent increase in the image formation speed indicates a need to increase the speed of toner replenishment from the toner replenishing device per hour. If the number of revolutions of the toner replenishing conveying member is increased to increase the speed of toner replenishment, the number of revolutions of the toner agitation member of which driving source is common to that of the toner replenishing conveying member is also increased. Besides, toner having excellent flowability (low accelerated aggregation degree) is used to respond to the increase in the image formation speed. Therefore, if the toner is excessively agitated by the toner agitation member, air excessively mixes into the toner, so that the toner is made in a floating state. If the toner is made in the floating state, an amount of toner per unit volume becomes small and a toner detection sensor cannot detect that toner remains. In particular, when a type of sensor that detects whether toner remains based on the size of a load applied onto a detection surface of the sensor, for example, a sensor of a piezoelectric vibration type is used, if the toner is made in the floating state as described above, the load applied onto the detection surface becomes small. Accordingly, the sensor makes erroneous detection that toner does not remain in the toner container although toner remains therein.
The toner detection sensor of the above piezoelectric vibration type is a sensor that detects whether a load is applied using the following principle. In the toner detection sensor, electrodes are provided on both surfaces of a plate-like piezoelectric ceramics. Then, if a load is applied in a state where an alternate signal is applied to the electrodes on both surfaces thereof to oscillate the electrodes, phase characteristics thereof change. When the toner detection sensor is used to detect a toner remaining amount in the toner container, one of the electrodes on both surfaces is set to be a detection surface. The toner detection sensor is arranged on a wall portion of the toner container such that the electrode faces an inner side. Then, toner is made direct contact with the detection surface to detect whether toner remains. Since the toner detection sensor has high sensitivity, if toner is kept adhered to the detection surface, the toner detection sensor makes erroneous detection that toner remains in the toner container although toner does not remain therein. The toner detection sensor makes such erroneous detection because the sensor detects the toner adhered to the detection surface. Therefore, it is desirable that the detection surface is regularly cleaned to scrape off the toner adhered to the detection surface.
In recent years, there is the need for reduction of a toner replenishing device in size in addition to an increase in the image formation speed. To reduce the device in size, a space occupied by each component that constitutes the device is required to be saved. Therefore, a space occupied by a toner container is also required to be saved. If the space occupied by the toner container is saved to reduce the device in size, the capacity of the toner container becomes smaller so that a toner volume housed therein is also reduced.
The present inventors used a toner detection sensor of the above-described piezoelectric vibration type to detect a toner remaining amount in a toner container of which capacity is smaller than that of the conventional one. Then, the inventors found that the sensor made erroneous detection that toner does not remain if toner remains in some cases. Furthermore, it was found that the erroneous detection possibly occurs not only in the above piezoelectric vibration type sensor that detects a load applied onto a detection surface of the sensor but in sensors of other types whose detection surface is desirably regularly cleaned to scrape off toner adhered to the detection surface. The sensors of other types include a detection sensor of a magnetic permeability detection type and a detection sensor of a transmitted light detection type, for example.
It is therefore an object of the present invention to provide a toner replenishing device capable of detecting a toner remaining amount accurately even if the capacity of a toner container is smaller than that of the conventional one when a detection surface of a toner detection sensor that detects the toner remaining amount in the toner container is cleaned and to provide an image forming apparatus including the toner replenishing device.